Certainly a few lapses are acceptable. My general philosophy with learning any language is, if you really need a word, you'll encounter it again sooner or later, so you can just hold off memorizing it until then. If you're a professional translator who needs to know words on the spot, that's one thing, but obviously that's not going to be the case wth a dead language like Latin.

What's much more important than memorizing a word list is being able to comprehend the majority of what you read -- understanding the flow of the sentence, the role the words play in it, etc. -- and if there happen to be any unknown words, you can either guess from context or look them up. You will need to know most of the words if you want to be able to read anything at an acceptable pace, but by no means every single one.